[llvm-exegesis] Allow benchmarking arbitrary code snippets.

Summary:

This is a step towards fixing PR38048.

Note that right now the measurements are given per instruction. We'll
need to give measurements a per code snippet and update the analysis (PR38731).

Reviewers: gchatelet

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D52041

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342947 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed